1. Field of the Invention
The present invention relates to multi-rate communication systems, such as the CDMA cellular telephone network, and more particularly to a method and means for providing a communication frame structure that can support a number of different physical layer data rates.
2. Prior Art
In existing multi-rate communication systems, such as the CDMA communication system using the IS-95 Standard, the communication messages or transmissions are transmitted using a frame structure having physical layers with different data rates. According to the IS-95 communications standard, transmissions have a physical layer frame size F, a frame period T, and a physical layer data rate R. Details of the many aspects of this technology can be found in the standards publications identified by IS-95A, TSB74, and ANSI J-STD-008, which are familiar to those of skill in the art and incorporated herein by reference.
One approach for varying the data rate is to keep the same frame size and vary the frame rate, e.g., frames may be generated every 5 ms, instead of every 20 ms as is typical, in order to increase the data rate from 9.6 kbps to 38.4 kbps. With this approach, however, the minimum air interface data rate is 4.8 kbps, rather than 1.2 kbps, which is not economical since, for instance, it wastes bandwidth. Additionally, various system timing parameters are based on the frame rate so that a change in the frame rate results in changing other system timing parameters, which changes are not desirable. Further, frames of data are interleaved over a single frame and, as it is desirable to maintain an interleave depth of 20 ms for good error correction, a frame of 5 ms, and thus an interleaving depth of 5 ms, would not provide adequate interleaving for good error correction.
Another approach is to keep the same frame rate and define different frame sizes. However, this may lead to very large frame sizes for high data rates, which results in a higher frame error rate (FER). Also, a large number of different frame types must be implemented to accomodate the various data rates in between the lowest and the highest data rate.
Thus, these two approaches to varying the data rate have limitations regarding efficiency.
Problem to be Solved:
In multi-rate communication systems, therefore, a frame structure is needed that can support a number of different physical layer data rates efficiently.
Objects of the invention:
It is an object of the present invention to provide a framing technique that overcomes the limitations in both the above-noted prior art schemes.
It is another object of the invention to provide a frame structure that can support a number of different physical layer data rates efficiently.
It is a further object of the invention to provide a framing technique that supports a large number of data rates using only a small number of different frame types.